A fluidized catalytic cracking (FCC) unit is the most important secondary processing unit in a refinery for cracking gas oil or residual feedstocks and is considered as the work horsepower of a refinery. FCC units operate either on maximum gasoline mode or the maximum diesel mode, which depends on seasonal product demand FCC units are gaining vitality now days, since it can be operated in high severity mode to produce high yields of light olefins than conventional FCC operation. The olefins produced in FCC unit are also an important source for petrochemicals.
The FCC unit comprises the reactor and regenerator, which are considered to be the core element of the fluid catalytic cracking unit. The preheated hydrocarbon feedstock is injected into the riser where it is vaporized and cracked into smaller molecules by contact with the hot catalyst from the regenerator. The hydrocarbon vapours fluidize the catalyst in the riser and the mixture of hydrocarbon vapours and catalyst flow upward to enter the reactor. The reactor is a disengaging vessel in which the cracked product vapours are separated from the spent catalyst. The separated spent catalyst flows through a steam stripping section to remove any hydrocarbon vapours before the spent catalyst returns to the regenerator. The spent catalyst having coke deposited on it is regenerated by burning off coke with air into the regenerator. The combustion of the coke is exothermic, which heats the regenerated catalyst and provides the heat required for the vaporization of the hydrocarbon feedstock and the endothermic cracking reactions that take place in the riser. The hot flue gas exits the regenerator after separation of entrained catalyst particles in the cyclones.
FCC units typically produce around 3-5 wt % propylene, depending on feedstock type, operating conditions such as riser outlet temperature, reactor pressure, catalyst-to-oil ratio and the type of FCC catalysts/additives. Usage of both ZSM-5 additive and increased operation severity increases the light olefin yield from the FCC unit at the expense of gasoline.
The prime source of ethylene and propylene is steam crackers. However due to the focus shift of naphtha crackers to ethane crackers, propylene production has suffered. Therefore, enhancing the propylene yield from FCC is one of the ways to meet the propylene demand in the market.
U.S. Pat. No. 4,980,053 describes deep catalytic cracking (DCC) process for catalytic cracking of VGO with mixed solid acid catalyst to produce propylene and butylene at very low weight hourly space velocity (WHSV) of 0.2 to 20 hr−1, catalyst/oil of 2-12 wt/wt and temperature of 500 to 620° C. The LPG yield obtained is 30 to 45 wt %. However, the drawback of the process is high dry gas yield (6-12 wt %). Further, high severity operation requires additional source to build coke on catalyst to meet heat balance requirements for riser and dense bed cracking. Also, the reactor size is more than conventional FCC reactor due to WHSV.
U.S. Pat. No. 5,846,402 describes the process for the production of high yield of LPG and light olefins in the range of C3 and C4 from various petroleum fractions including residues through very high severity catalytic cracking using a metal resistant mixed catalyst system. The reactor operates at a WHSV of 40-120 hr−1, catalyst to oil of 15 to 25, Pressure of 1 to 4 kg/cm2 and temperature of 530 to 600° C. The LPG yield obtained is 40-65 wt %, propylene make is 40-50 wt % in LPG and ethylene is 50 wt % in Dry Gas. However, the major drawback of the process is the low coke production in hydrotreated VGO feed. Thus, additional coke is needed to meet the heat requirements of riser. The process is riser cracking based and suitable mainly for heavy residual petroleum feedstock and it is difficult to operate at lower pressure with hydrotreated VGO feed, which is desirable for light olefin selectivity. The patent is silent on cracking C4s, which is essential for increasing propylene yield. The patent discloses the usage of single reactor, whereby bottoms yield is difficult to be reduced to the minimum level.
U.S. Pat. No. 6,656,346 describes a process to crack heavy oil fraction by contacting oil with a catalyst mixture of 60 to 95 wt % base USY zeolite catalyst and 5 to 40 wt % shape selective zeolite in the range of 580 to 630° C., catalyst to oil of 15 to 40 wt/wt and contact time of 0.1 to 1 secs. However, the unit configuration is totally different from conventional riser cracking and comprises of combustor type regenerator for catalyst regeneration and downflow riser for catalytic cracking. The high severity operation with high catalyst to oil ratio for paraffinic gas oil feed (which is more selective towards light olefins) needs additional source of heat to meet heat balance. Also there is huge requirement of catalyst due to high cat-oil ratios.
U.S. Pat. Nos. 7,611,622, 5,702,589, US 2014/0014555A1 and U.S. Pat. No. 6,093,867 discloses different methods for recycling/injecting of naphtha and C4 streams in a separate riser or in riser bottom as well as stripping zones to increase light olefin yields.